The accurate measurement of DC biopotentials for sensing or screening for disease, injury or bodily functions is very difficult to accomplish, for the DC potentials to be sensed are of a very low amplitude. Due to factors such as the low DC potentials involved and the innate complexity of biological systems, the collected data signals tend to include a substantial volume of noise which makes accurate analysis difficult. Also, biological systems are notorious for their complexity, nonlinearity and nonpredictability, and wide variations from the norm are not uncommon. For example, DC biopotential signals tend to drift over time, so that if signals are not sensed and analyzed with some rapidity, signal errors due to drift occur.
For the accurate measurement of DC biopotentials for disease diagnosis and screening, electrode and electrode circuit characteristics and electrode placement become important. If an electrode ceases to make good contact with a subject during a measurement cycle, an erroneous indication may result.
Factors such as small DC offset potentials in the low millivolt range, which may have little effect on an AC biopotential measurement, such as ECG measurement, can destroy the accuracy of a DC biopotential measurement. For screening applications where many sensing electrodes are used, it is often critical for electrode characteristics to be uniform, for accurate electrode spacing to be maintained and for DC offsets to be substantially eliminated.
Many DC biopotential sensing electrodes are packaged in a pre-gelled state wherein an electrolytic paste or gel is packaged as part of the electrode. The gel may be located in a central gel reservoir consisting of a molded cup, or it may be contained in a dye-cut hole in a foam which encapsulates a gel saturated open cell compressible foam column. In most instances, the pre-gelled electrodes are sold ready for use with an electrically conductive material such as metal or a metal chloride in contact with the electrolyte gel.
A pre-gelled electrode system is generally not a battery by itself, but forms a part of a battery-system consisting of two or more electrodes placed on the body. In such a system, a complex battery is formed consisting of many interactive components including the electrode material (frequently silver/silver chloride), the electrode gel, internal body chemistry and external skin conditions, skin preparation, temperature, air condition and chemistry, etc. Obviously, some of these factors are not subject to control, but in order to get the best data possible, especially in instances where DC biopotentials are of interest, artifacts, such as DC offsets, should be reduced to the lowest level. Most pre-gelled electrodes when introduced in the battery system outlined above contribute some unwanted DC voltage (polarization effect) to biopotential measurements. It is important to lower the possibility of such DC artifacts occurring to a degree sufficient to preclude them from having a substantial adverse effect on biopotential measurements.
The design and performance characteristics for an effective DC biopotential electrode are different from those of electrodes designed for measuring alternating current (AC) signals such as those used with electrocardiology (ECG) and electroencephalography (EEG). For example, U.S. national standards for single use ECG electrodes allow the DC offset of an electrode pair (i.e., the spurious DC current generated by electrochemical interactions between electrode components) to be as high as 100 millivolts (ANSI/AAMI standard). Since effective use of DC signals for cancer diagnosis requires discrimination at the one millivolt level, standards for ECG electrodes are grossly excessive. ECG electrodes are intended for AC measurements which are not significantly affected by DC offset voltages in the electrode to the degree that DC biopotential measurements are adversely affected by such offset voltages. The traditional view taken in the manufacture of ECG pregelled electrodes is that to reduce DC offset, one must sacrifice AC impedance, and since a low AC impedance is most important in an ECG pregelled electrode, the DC offset voltage is tolerated. However, for highly accurate DC biopotential measurements, both the DC offset potential and the AC impedance for the electrode must be low.
If a pre-gelled electrode array is to be used effectively for disease detection, such as breast cancer screening, the array will require a relatively large number of spaced electrodes to cover substantially the entire surface of the breast. Not only must each of these electrodes be free from error causing offset potentials before use, but the electrodes must maintain contact with the curved surface of the breast without movement during the screening procedure and must maintain a predetermined array formation with specified electrode spacing. Consistent location and orientation of the electrical channels connected to the respective electrodes must also be maintained to prevent incorrect connection to the electrodes and to maintain positive contact between the electrodes and the electrical channel leads therefor.
The key to effective measurement and analysis of direct current skin potentials is absolute maintenance of signal integrity from the skin surface to the signal processing components of the measuring unit. This is especially critical due to the inherent low amplitude of biologic DC potentials. At any point in the electronic path from the skin sensing electrode to the measuring unit, potential exists for noise to intrude upon signal, thereby degrading diagnostically useful information.
In a DC biopotential sensing electrode, the connection between the electrode and the electrode lead which provides an output signal from the electrode to a measuring instrument is extremely important. Disposable DC biopotential electrodes normally include a sensor disc having a projecting button type connector with an enlarged head portion which is engaged by an electrical lead connector unit. To minimize the AC impedance of the electrode, the sensor disc and button connector are formed from a plastic body which is uniformly and entirely coated with a very thin layer of a conductive metal such as silver. This metal layer has a thickness within a range of 0.5 mil to 1.5 mil, and a break or disruption of this layer can degrade the DC biopotential sensed by the electrode.
Disposable medical electrodes normally are connected to an electrical lead by a snap connector in the form of a cup shaped female receptacle which is pressed downwardly over the button connector of the electrode sensor disc once the electrode is positioned on a subject. Although these snap connectors are effective when used with most A.C. disposable electrodes, such as ECG electrodes, they can be problematic when used with D.C. biopotential electrodes. With D.C. biopotential electrodes, a connector which is moved in engagement across the surface of the button connector during the connection process is likely to abrade or disrupt the thin conductive surface of the button connector during engagement. Also, since D.C. biopotential electrodes are presently used primarily for the detection of, or screening for breast cancer, the downward pressure required to engage a snap connector can cause pain or discomfort while also resulting in undesirable spreading of the electrode gel or electrolyte. Also, with snap connectors, patient movement can produce tension on the electrical lead from the electrode and movement or disengagement of the electrode from the skin of a subject.